Timing the first irrigation

15 December, 2017

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After planting and establishing a plant stand, the next major management decision is the timing of first irrigation – and with many growers experiencing a dry start this season, it’s particularly important to get the balance right.

The first in-crop irrigation plays an important role in setting the crop up to generate strong growth during the flowering period, particularly with high retention Bollgard crops. A well timed irrigation around first flower will also help the crop to reach desired targets such as fruit retention, fibre quality and boll weight.

First irrigation scheduling is a difficult balancing act between not waterlogging and stressing the plant, while ensuring stored water in the soil profile is fully explored by the developing root system. Mistiming or delaying first irrigation for too long will incur yield penalties due to the impact of water stress on plant development.

Actual timing of first irrigation will vary depending on seasonal conditions and in-crop rainfall; where excessively hot, dry or windy conditions can bring first irrigation forward, while wet and cool conditions can delay it. Fields with lighter soils or compaction will generally require an earlier first irrigation.

There are two main options for growers with limited water – to stretch out the irrigation interval, or to water normally and potentially cut off water at the end of the crop. The success of each option will depend on summer rainfall, and the crop’s response to stress will vary depending on its growth stage.

When making the decision to irrigate, growers should be sure to:
• Monitor soil moisture, root extraction patterns, daily water use and plant vigour.
• As a rule of thumb, irrigate at 50 per cent available soil water within the root zone.
• Check weather forecasts as hot and dry cool or wet weather near the time of first irrigation can be detrimental to crop growth and water use efficiency.

By Chris Barry, Extension & Development Agronomist – Darling Downs & Central Queensland